Electron spin–lattice relaxation mechanisms of rapidly-tumbling nitroxide radicals

Abstract

Electron spin relaxation times at 295K were measured at frequencies between 250MHz and 34GHz for perdeuterated 2,2,6,6-tetramethyl-4-piperidone-1-oxyl (PDT) in five solvents with viscosities that result in tumbling correlation times, τR, between 4 and 50ps and for three 14N/15N pairs of nitroxides in water with τR between 9 and 19ps. To test the impact of structure on relaxation three additional nitroxides with τR between 10 and 26ps were studied. In this fast tumbling regime T2-1∼T1-1 at frequencies up to about 9GHz. At 34GHz T2-1>T1-1 due to increased contributions to T2-1 from incomplete motional averaging of g-anisotropy, and T2-1-T1-1 is proportional to τR. The contribution to T1-1 from spin rotation is independent of frequency and decreases as τR increases. Spin rotation dominates T1-1 at 34GHz for all τR studied, and at all frequencies studied for τR=4ps. The contribution to T1-1 from modulation of nitrogen hyperfine anisotropy increases as frequency decreases and as τR increases; it dominates at low frequencies for τR>∼15ps. The contribution from modulation of g anisotropy is significant only at 34GHz. Inclusion of a thermally-activated process was required to account for the observation that for most of the radicals, T1-1 was smaller at 250MHz than at 1–2GHz. The significant 15N/14N isotope effect, the small H/D isotope effect, and the viscosity dependence of the magnitude of the contribution from the thermally-activated process suggest that it arises from intramolecular motions of the nitroxide ring that modulate the isotropic A values.